Since nitride semiconductors have a band gap in the far infrared to ultraviolet wavelength region, they are promising as a material of light emitting or light receiving devices in that region. Also, the nitride semiconductors have a wide band gap and have a large breakdown field and a high saturation electron velocity. For that reason, the nitride semiconductors are also very promising as materials of electronic devices with high-temperature, high output power and high frequency operation. Further, since the nitride semiconductors do not contain arsenic (As) and phosphorus (P), as compared with GaAs based or InP based semiconductors which have hitherto been utilized, they have a merit that they are harmless against the environment and are expected as a semiconductor device material in the future.
As a substrate for epitaxial growth of nitride semiconductor having such excellent characteristics, any material having a lattice constant and a coefficient of thermal expansion equal to those of the nitride semiconductors has not been available yet. For that reason, sapphire, SiC, or Si is mainly used as the substrate.
For epitaxial growth of GaN, AlN, InN and their alloyed crystals, a sapphire substrate has hitherto been mainly used. However, there are lattice mismatch of 11 to 23% and a difference in the coefficient of thermal expansion between the sapphire substrate and the nitride semiconductor. Accordingly, if the nitride semiconductor is grown directly on the sapphire substrate, the three-dimensional growth occurs so that the flatness of the surface in an atomic level becomes worse. For that reason, there was a problem that the nitride semiconductor grown on the sapphire substrate has a number of crystal defects.
In the case of the epitaxial growth of a nitride semiconductor on the sapphire substrate, it has been reported that crystallinity of GaN was improved by a method using a buffer layer. Its technologies will be described below.
The first is a growth method of GaN using a low-temperature AlN buffer layer (see the following Non-Patent Document 1). This method is as follows. The sapphire substrate was heated up to the temperature around 1000° C. for surface cleaning in metalorganic vapor phase epitaxy system etc., the temperature was then once dropped. Next, a low-temperature AlN buffer layer was deposited at around 500° C., and the temperature is again raised. Then, GaN was grown at around 1000° C. The ALN buffer layer deposited by this method is amorphous and the islands were formed during the temperature rising step due to the solid phase growth of amorphous AlN. As a matter of course, the island shape to be formed varies depending upon the atmosphere in the growth system (apparatus)or the temperature rising rate during the temperature rising. At the beginning of growth of the GaN layer at high temperatures, this island becomes a nucleus, whereby the GaN layer undergoes crystal growth. During that crystal growth, flattening of the GaN layer advances due to the coalescence. GaN undergoes two-dimensional crystal growth on the flattened GaN layer.
The second is a growth method of GaN using a low-temperature GaN buffer layer (see the following Non-Patent Document 2). This method is as follows. The sapphire substrate was heated up to the temperature around 1000° C., the temperature was then once dropped. Next, a low-temperature GaN buffer layer was deposited at around 500° C., and the temperature is again raised. Then, GaN was grown at around 1000° C. Since GaN is decomposed easily at high temperature as compared with AlN, the nucleus formation in the temperature rising step is not always the same as in the case of AlN, but the subsequent growth process is substantially the same.
Incidentally, in the crystal growth of nitride semi-conductors other than GaN, the same methods as in those described previously are applicable, too. For example, in growth of Al1−xGaxN (0≦x<1) or In1−xGaxN (0≦x<1) crystals, a low-temperature GaN buffer layer is deposited on the sapphire substrate, then GaN, and Al1−xGaxN or In1−xGaxN was grown. In particular, a method of growth of Al1−xGaxN crystals is described in the following Non-Patent Document 3.
As described previously, in all of these growth methods, the buffer layer was aimed to achieve lattice matching with the GaN layer, but lattice matching with the substrate was not taken into consideration.
Also, even if the buffer layer is deposited at a low temperature, the low-temperature buffer layer is amorphous and solid phase growth occurs at the time of temperature rising. For that reason, the lattice mismatch between the buffer layer and the substrate still exists, it is difficult to effectively suppress the generation of crystal defects, and threading dislocation of 109 to 1010 cm−2 exists usually. It is well known that this dislocation deteriorates the characteristics of a fabricated device. For example, shortening of the life of laser and an increase of leak current and a lowering of breakdown voltage of the device. Also, diffusion or segregation of impurities may possibly be promoted due to the existence of the dislocation. Accordingly, reducing the dislocation density in the nitride semiconductor layer is very important for improving the device characteristics, realizing devices which have not been attained so far due to influences of the dislocation and enhancing the controllability in fabrication of a device structure in crystal growth.
Accordingly, the invention is aimed to provide a substrate for growth of a nitride semiconductor capable of obtaining a high quality nitride semiconductor crystal layer.
Non-Patent Document 1:
H. Amano, N. Sawaki, I. Akasaki, and Y. Toyoda, “Meta illustrated in the foregoing embodiment, the GaN buffer layer 9 (thickness: 1 μm), the n+-type GaN subcollector layer 10 (thickness: 1 μm), the n−-type GaN collector layer 11 (thickness: 0.5 μm), the p-type GaN base layer 12 (thickness: 0.08 μm), the n−-type Al1−xGaxN emitter layer 13 (0 ≦x<1) (thickness: 0.05 μm), and the n+-type GaN contact layer 14 (thickness: 0.1 μm) were grown by the metalorganic vapor phase epitaxy. In this case, the growth sequence is a method in which the substrate 6 for growth of nitride semiconductor was introduced into a growth furnace, the temperature was then raised to the growth temperature (1,000° C.) under an ammonia atmosphere, and a source material gas was supplied. Trimethylgallium, trimethylaluminum and ammonia are used as the source materials. For dopant of n-type impurities, a Si was used. For dopant of p-type impurities, Mg was used. A mesa structure was prepared by etching, and ohmic electrodes, i.e., the collector electrode 15, the base electrode 16, and the emitter electrode 17, were formed on the each exposed layers by means of electron beam metal deposition. In a collector current-collector voltage characteristic in common emitter configuration of a fabricated transistor, current gain of approximately 100 was obtained, and the breakdown voltage was increased to approximately 200 V with a reduction of the dislocation density as described already being reflected.